Authors:

Determining the onset conditions for magnetic field growth in
magnetohydrodynamics is fundamental to understanding how
astrophysical
dynamos such as the Earth, the Sun, and the galaxy self-generate
magnetic fields. The role of turbulence in modifying these onset
conditions is studied in the Madison Dynamo Experiment. A turbulent
flow of liquid sodium, composed primarily of two counter-rotating
helical vortices, is generated by impellers. Laser Doppler
velocimetry
measurements of the flow in an identical-scale water experiment
demonstrate that the turbulence is isotropic, though not homogeneous,
with particularly long-lived eddies in the shear layer between
the two
flow cells. The magnetic field induced when an axial field is applied
shows intermittent periods of growth and has a spatial structure
consistent with the fastest growing magnetic eigenmode predicted by a
laminar kinematic dynamo model of the mean
flow.\footnote{Nornberg {\em
et al.}, Phys.\ Rev.\ Lett., in press (2006),
physics/0606239.} Turbulent fluctuations of the velocity field change
the flow geometry such that the eigenmode growth rate is temporarily
positive, thus generating the magnetic bursts. It is found from
ensemble averaging that the bursts gain strength and frequency
with increased impeller rotation rate, though they become shorter so
that each burst remains a rare, random event.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2006.DPP.BP1.95